1. The Field of the Invention
The present invention relates to apparatus for reducing the particle size of natural pozzolanic materials.
2. The Relevant Technology
A pozzolan is a material which, when combined with calcium hydroxide, exhibits cementitious properties. Because of its properties, pozzolans are commonly used as an admixture to Portland cement concrete mixtures to increase the long-term strength of the concrete and provide other beneficial properties. For example, when added to concrete, pozzolanic materials can improve the compressive strength, bond strength, abrasion resistance and other properties of the concrete.
Pozzolans are known to be slower reacting than Portland cement, primarily due to their lower content of tricalcium silicates. When used in large quantities, pozzolanic material tend to retard early strength development. To increase the reactivity of pozzolanic materials, pozzolans can be ground to create smaller pozzolanic particles. Grinding a pozzolan reduces the particle size and increases the surface area, which increases the reactivity of the particles.
To achieve a desired particle size, pozzolans are typically crushed in a grinding mill such as a ball mill. A ball mill is a horizontal cylinder partly filled with steel balls (or occasionally other shapes) that rotates on its axis, imparting a tumbling and cascading action to the balls. Material fed through the mill is crushed by impact and ground by attrition between the balls. The grinding media are usually made of high-chromium steel. The smaller grades are occasionally cylindrical (“pebs”) rather than spherical.